Constrain vertex to its face
One thing you can try is to scale selected vertices while locking normal Z axis.

To do so, enable Normal transform orientation with Alt+Space or from viewport header. Select vertices, enable Scale tool with S and press Shift+Z to lock movement along global Z axis, then Shift+Z once again to lock normal Z axis. After that move your mouse to change amount of effect.



It's also possible to move vertices as well while constraining normal Z axis making them roughly stay on the same surface.